Gas discharge plasma processing has become widely used to process integrated circuits, and has certain characteristics that render it superior to traditional etching techniques. Etching is a technique by which openings in a developed resist mask are transferred into the permanent layers of an integrated circuit substrate. The earlier technique of chemical wet etching has become widely supplanted by gas discharge plasma etching, especially for precision applications. The reason for this replacement is that chemical wet etching is a nondirectional (isotropic) process which etches, not just vertically into the substrate, but also laterally under the photoresist mask to create an unwanted undercutting effect in the permanent layers of the integrated circuit substrate. For example, if the substrate under a photoresist opening is etched to a depth of one micron, this undercutting adds one micron to each side of the transferred profile. This enlargement, a factor of three for a one micron wide opening, is too large and unacceptable for the current micron and submicron technology.
In contrast, plasma etching uses ions in a gas discharge to transfer the resist pattern directionally (anisotropically) into the substrate. As a result, the undercutting effect is zero and there is no unwanted enlargement of the etched pattern. Because of this advantage, gas discharge plasma etching processes have become commonplace, especially where dimensions, profile considerations, and shaping are critical.
In a simple plasma etching process, a flow of low pressure gas is admitted to a vacuum containment chamber and subjected to a radio frequency excitation. The radio frequency excitation is provided by an electrode that is housed within the vacuum containment chamber and is connected to a radio frequency power source for generating an electrical field. Electron impact with the gas molecules produces a plasma which contains ions, a larger number of reactive neutral (radical) species, and more electrons to sustain the discharge. When exposed to the plasma generated by this electrode, the wafer is bombarded by ions which strike perpendicular to its surface.
High rate etching is such conventional reactors invariably necessitates high power excitation; which may be radio frequency power, resulting in typical ion energies of 400 electron volts (eV). From a microscopic point of view, an ion impacts the etching surface at many hundreds of electron volts and, by collision cascade, spreads a large share of its energy far too deeply to be efficiently utilized for etching at the surface. The excess energy is dissipated by generating heat which may destroy the resist and produce damage in the substrate. The problem is aggravated if a higher etching rate is required, because increasing power to the electrode increases not only the etch rate but also the energy of the incident ions. Thus, there is a major lack of proportion between the energy of the ions, which is often several hundred eV, and the required chemical activation energy of the item being processed, which is typically less than 0.2 eV.
The magnetron principle may be used to reduce the ion energy to the 100-200 eV range at high power and low pressure. Systems making use of this principle are disclosed in U.S. Pat. Nos. 4,581,118, 4,525,262, and 4,442,896. The multiple field magnetron herein disclosed further reduces the ion energy to the 3 eV to 70 eV range.
Accordingly, it is an object of the present invention to provide a method and apparatus for magnetron gas discharge processing utilizing multiple, independent electric fields.
It is a further object of the present invention to provide a method and apparatus for magnetron gas discharge processing that generates, using a multi-part cathode, multiple independent electric fields around the cathode.
It is a further object of the present invention to provide a method and apparatus for magnetron gas discharge processing that increases the ion flux density for a high etch rate while reducing the ion energy to minimize damage and heat generation.
It is a further object of the present invention to provide a method and apparatus for magnetron gas discharge processing that increases the ion flux density while independently controlling ion energy.
It is a further object of the present invention to provide a method and apparatus for magnetron gas discharge processing that achieves a high etching rate at low pressure while reducing the ion energy.
These and further objects of the invention, as will become apparent from the detailed description, are accomplished with the present invention.